Summary: The goals of this project are to characterize the molecular identity of memory lymphocytes and to elucidate the mechanisms underlying immunological memory. We study human peripheral blood memory and naive CD4+ T cells as a model because they are phenotypically and functionally well characterized. Memory CD4+ T cells express CD45R0 while naive CD4+ T cells express CD45RA, which was used as the basis for isolation of these two subsets. To assess the global gene expression of memory cells at rest and after activation in comparison with na?ve cells using cDNA microarray, we analyzed over 58,000 unique human cDNA clones and found that memory and naive CD4+ T cells expressed about 22-27% of total clones. Overall, memory and naive CD4+ T cells express similar number of genes at rest and after activation. Genes that inhibit cellular proliferation are highly expressed in resting cells but are uniformly down-regulated after stimulation in both memory and naive CD4+ T cells. In contrast, the expression of activation-induced genes displays a gradient pattern in which the mRNA levels correlate with the degree of activation, and differentiate memory from naive cells. Together, our results defines the transcriptional nature of memory CD4+ T cells at rest and after activation, and identify a molecular mechanism that differentiates the response of memory CD4+ T cells from naive CD4+ T cells. In the process of global analysis, we have identified over 170 known genes whose expression was significantly changes after activation. Recently, we have extended the study of mRNA levels analyzed by cDNA microarray to conventional Northern and RT-PCR, as well as protein levels of selected genes. Currently, we are studying the temporal changes of these activation-related genes and their function in memory and naive CD4+ T cell activation.